Unable to install extensions from local .xpi files

🐞 Bug Report

Reproduction steps

1. Navigate to Settings -> About -> About Tor Browser (or About Tor Browser Alpha).
2. Tap the Tor logo 5 times until you see a notification that the Debug menu is enabled.
3. Navigate back to Settings -> Advanced -> Install extension from file, and try to install an extension (I tested with Dark Reader's latest darkreader-firefox.xpi file from here.

Expected behaviour

The user should receive a prompt to install the add-on.

Actual behaviour

Nothing happens.

Bookkeeping

• Browser version: Confirmed on 14.5.3 & 14.5a6
• Browser channel:
  • Release
  • Alpha
  • Nightly
• Distribution method:
  • Installer/archive from torproject.org
  • tor-browser-launcher
  • homebrew
  • other (please specify):
• Operating System:
  • Windows
  • macOS
  • Linux
  • Android
  • Tails
  • Other (please specify):
• Operating System Version: Android 15 (GrapheneOS)

Have you modified any of the settings in about:preferences or about:config? If yes, which ones?

No.

Do you have any extra extensions installed?

No.

Troubleshooting

Does this bug occur in a fresh installation?

Yes.

Is this bug new? If it is a regression, in which version of the browser did this bug first appear?

Not sure when specifically this was introduced, but I believe it's related to !1240 (merged). We (IronFox) also use a slightly modified version of this patch, and are also impacted by this issue (I was made aware of it due to a report from a user.

Does this bug occur in the Alpha release channel?

Yes.

Does this bug occur in Firefox Rapid Release?

No.

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